Fabrication of tubing



Aug. 28, 1945- c. E. SLAUGHTER FABRICATION 0F TUBING Filed Sept. 11, 1942 INVENTOR. Cfiar/as f. J/cuy/ner Patented Aug. 28, 1945 FABRICATION OF TUBING Charles E. Slaughter, New Canaan, Conn., assignor to Extruded Plastics, Inc., Norwalk, Conn., a corporation of Connecticut Application September 11, 1942, Serial No. 458,051

16 Claims.

This invention relates to the art of fabricatin tubing. It is more particularly concerned with a method of fabricating tubing from thermov plastic materials.

It is common practice to form tubing from thermoplastics by a continuous process usually termed extrusion. The extrusion of tubing involves the feeding of a granular thermoplastic into one end of a hollow cylinder wherein it is heated to soften it. The cylinder is. provided with a suitable internal screw member having flights thereon for positively conveying the madeform materially.

- of varying wall thickness.

This method is subject to the disadvantages of producing tubing which is out-of-round and Another disadvantage of such method lies in the fact that it is almost impossible to produce tubing of any sub-- stantial size, e. g., three-eighths inch internal diameter. or larger, which does not vary in crosssection along its length over a range as much as plus or minus twenty thousandths of an inch or more.

It is the principal object of this invention to provide a method whereby tubing of substantially uniform cross-section can be produced.

Another object of the invention is to provide a method whereby. tubing of substantially uniform cross-section in a variety of shapes can.

be produced from a single extrusion die.

A further object of the invention is to provide a method whereby tubing of a predetermined cross-section can be produced at a relatively high rate.

Other objects will become apparent from the following description.

I have now found that the foregoing objects can be accomplished by expressin a heated ther moplastic through a die to form a tube while supporting the tube internally by introducing a fluid thereinto, in the usual manner, and thereafter drawing the so-formed tube, while'it is still relatively plastic, successively through a plurality of draw plates or members spaced apart, each of said members having a similarly shaped opening slightly larger than the opening in the preceding member. Preferably, the tube is then drawn through a draw plate or member provided with an opening of the exact external shape desired in the finished tube, said member being disposed at other-than a right angle to the line of draw.

The invention will best be understood with reference to the accompanying drawing. wherein:

Figure 1 is a side sectional elevation illustrating suitable apparatus with which the invention can be practiced.

Figure 2 is a plan view of the apparatus shown in Figure 1.

Before proceeding with a description of the drawing, it is thought the invention can best be explained by mentioning some of the principles which appear to determine the best method.

of practicing the invention. I have found that it is extremely important to contact at the same instant the entire periphery of the extruded tubing, while still very hot, with a coolant. If this is not done, the tubing shrinks more in one portion than in another and the strains set up therein cause it to become out-ofround. A 'second extremely critical feature of the invention is that the tubing must be given its final draw through a plate set at other than a right angle to the line of draw. The temperature of the tubing at the instant it is pulled through the final draw plate and the rate of travel of the tube are very important. The tubing must be hard enough to hold to shape as it emerges from the final draw plate but .must be slightly plastic as it enters the plate so that it can be shaped therethrough. A third important feature in connection with the operation of the invention resides in the maintenance of a thin film of fluid coolant, whether in liquid or gaseous form, between the metal of each of the preliminary draw plates and the tubing drawn therethrough. The presence of such a film is provided for by the taper of the openings of the preliminary draw plates and precludes seizing such as would normally occur on contact between the plastic and the dry, hot metal of the draw plates. An exception to the foregoing is made with respect to the exact point of contact where the tubing enters the first draw plate and even here the pos sible existence of a cushioning film of gaseous coolant is appreciated.

Referring now to the drawing, the apparatus shown in Figures 1 and 2 consists essentially of a plurality of preliminary draw plates 3, 4 and 5. The plates are separated from each other by split spacing rings 6 and I. The plate 5 forms apart twenty thousandths of an inch. The openings in the three plates were arranged in the line of draw and were gradually enlarged at an angle of from the point of entry of the tubing into the first draw plate to the point of its exit 2 asaacao the end of a trough I containing water ordinarthrough the three preliminary draw plates. The iiy supplied at a temperature of about 55 F. air pressure in the tubing was sufficient to en- Tubing 9 is expressed through the orifice of an large it slightly after it emerged from the third extrusion die (not shown) and expanded somedraw plate and the enlarged tubing was then what by air forced thereinto in the cross-head 5 drawn through a plate set at an angle of 45 to holding the extrusion die. The expanded tubthe line of draw. The opening in this final draw ing 9 is drawn down and enters the opening in plate was exactly three-quarters inch in diamthe first draw plate 3 and thereafter passes eter. in the line of draw. The tubing was exsuccessively through similarly shaped openings truded at a rate of fifty pounds per'hour and was in each of the succeeding draw plates 4 and-I. drawn through the final draw plate at a speed The opening through the set of draw pla es 3. of fourteen feet per minute. In this manner, 4, and 5, constitutes a frustum of a cone, the thousands or feet of tubing have been drawn taper b in pr f r ly at n an le of about which were not measurably out-of-round and Th h tube 9 enters the Opening n e pr which varied in diameter less than plus or minus liminary draw plates at the point of least diam- 5 three thou andth of an inch. eter of the frustum and effectively prevents 0001- A referred embodiment of the invention as ing liquid from running out around the tu ing herein described includes the use of a die of through the opening in the first draw plate I. larger diameter than that of the finished tubing At the same time, owing to the increasing diamwhich expedites the maintenance of contact beeter of the open ngs through th draw plates 3. tween the tubing and the first draw plate thereby 4, and 5, water flows from the trough around the preventing the escape of the coolant. This relatubing and a film of water is interposed between tionship of die diameter to tubing diameter is e u n an h m al of the plates 4 and 5. largely determined by wall thickness of the tub- Because the spacing rings 6 and 1 are split and ing under construction and is inversely proporopen at the top and bottom water flows through tional thereto. Thus the heavier the tubing wall, the opening in the rings 8 and 1 at he o tom the smaller the die diameter in comparison to thereof, which permits a steady fiow of coolant that of the first draw plate and conversely. For to contact with the relatively hot plastic entering example, on very heavy wall tubing, 8. 8., t at the draw p having three quarters of an inch outside diam- Sllffieieht air may be oduc d n e tu eter and one quarter of an inch diameter hole, in 9 80 that i is r pan d somewhat after a dieas little as twenty thousandths of an inch pa in throu h preliminary draw plates 3, 4. larger in diameter than the first draw plate may and 5. This expanded tubing i0 is then drawn be employed. With thinner walled tubing, the through n pe in in he final w p e l die may have a diameter much larger than that which opening is the xa Size of the finished of the first draw plate. A contributing factor shape r d. The final dr w p e II is set at in the establishment of the above described proother than a right angle to the line of draw of portionality of diameters 1 t djmculty encounthe tubing 9 in such manner that one side of the tered in compress-drawing heavy wall tubing as tubing 9 is no longer in contact with the draw compared t thin wall tubing. p a II when the p os Side Of the tube @011- The invention is applicable to the fabrication tacts the draw plate II. It is convenient to supof tubing from a wide variety of thermoplastics port the w pl e I y a tubular member such as cellulose esters and ethers, polyvinyl esioined to the draw plate The said tubular ters and halides, certain polymeric vinylidene member I2 is provided with an opening through compounds and .copolymers thereof with other which water or other cooling liquid may fiow from p lymerizable materials, polyamide resins, and. in the main supply thereof in the trough into said general, all plastics which can be extruded and member i2 and thence into openings in the prewhich can be hardened by chilling for a suitable liminary draw plates 3, 4, and 5. Suitable takeperiod 01' time at a temperature below the exup means (not shown), for example, a large trusion temperature. driven winding reel, are provided to pull the tub- The temperature of the cooling liquid employed ing through the shaping means hereinbefore deto set the plastic and the rate of draw of the scribed. tubing are inter-related functions which must be As a specificexample illustrating the practice carefully adjusted .to give proper results. Coolof the invention in apparatus of the type shown ing liquids other than water may be employed. in the drawing, the fabrication of three-quarter e. g.. glycerine, hydrocarbon oils, et cetera, so long inch outside diameter tubing with a one-sixteenth as the coolant is substantially nonreacting with inch thick wall will be described. The tubing the particular plastic being drawn. Although it was made from Tenite II (a cellulose acetobutyis preferable to employ a liquid coolant, vario s rate plastic). This material was extruded at ap- 50 gases inert to the plastic being fabricated can be proximately 400 F. through an extrusion die with employed, particularly where the tubing size is an annular orifice one inch in outside diameter relatively small and the wall thickness is n o and with a wall thickness of seventy thousandths reat, of an inch. Sufilcient air was introduced into the A herei us d, th term tube or "tubing" tubing o pand it to an outside diameter 0 refers generically-to hollow elongated articles of about n n nei hth in he Th tubing uniform cross-section and is not to be construed was pulled into BI Preliminary drew P t having as restricted only to such articles of circular sec- & ir la p g of eighty hundre ths of n tion. For example, hollow articles of square, inch diameter at the point of entry f th u n triangular, or hexagonal cross-section can be Three ina y draw pla es w r us d. p ed 7 formed readily according to the present invention.

I have drawn square tubing three quarters of an inch on a side with a one-sixteenth inch-wall of uniform cross-section from tubing extruded through a circular annular orifice. This is of great advantage since it eliminates the necessity for having a special extrusion die for each shape of tubing desired. Such dies are frequently expensive, especially those for the extrusion of plastics which require heating means therein.

The angle of taper of the opening through the.

preliminary draw plates has been mentionedhereinbefore as While this appears to be the optimum angle, according to the experiments which I have performed, the invention is not restricted thereto and other angles of taper may be used, for instance from about 2 to about depending upon the thickness of the draw plates,

the thickness of the wall section of the tubing, the

particular plastic being worked, the speed of drawing, et cetera. Likewise, the angle at which the final draw plate is set with respect to the line of draw may vary from that hereinbefore specifie cally mentioned. It is only necessary that the tubing be subjected to force on one side before being contacted with the draw plate on the opposite side. Depending upon the thickness of the 'plate and the size of tubing being drawn, this angle may be from about 10 to about 80, preferably from 30 to 60.

While the foregoing description provides for passing the tubing through a final draw plate providedwith an opening of the exact external shape and diameter as that desired in the finished article, a leeway of up to five thousandths of an inch is permissible in the diameter of the opening of the final draw plate. Such difference can often be compensated for by control of temperature, rate of drawing, and other operating factors to obtain tubing within tolerance limits and satisfactory for most practical purposes.

I claim:

1. In a method for the fabrication of tubing,

. the steps which include: expressing a heated thermoplastic through a die to form a tube while supporting the tube internally by introducing a fluid thereinto; drawing said tube successively through a plurality of preliminary draw plates spaced apart, each of said plates having a similarly shaped opening slightly larger than the opening in the preceding plate, while said tube is still relatively plastic, and, thereafter drawing said tubethrough a final draw plate provided with an opening to give the exact external shape desired in the finished tube, the plane of said opening being at other than a right angle to the line of draw.

2. In a method of fabricating tubing, the steps which include: drawing extruded thermoplastic tubing While still relatively plastic through a plurality of preliminary draw plates spaced apart,

each of said plates having an opening of similar shape and slightly larger than the opening in the preceding plate; 'fiowing a liquid coolant countercurrent to .the direction of draw through the opening in each of said plates, except that with which the tubing is first contacted, to surround completely said tubing with the coolant; and, thereafter drawing said tubing through-a final draw plate provided with an, opening to give the external shape desired in the finished tubing, the plane of said opening being at other than a right angle to the line of draw, and the said draw plate being substantially immersed in a cooling fluid.

3. In a method of fabricating tubing, the steps which include: drawing extruded thermoplastic tubing while .still relatively plastic and internally supported by fluid pressure through a plurality of preliminary draw plates spaced apart, each of said plates having an opening of similar shape, the channel formed by the openings in the series of said plates being flared at an angle of about 5;

flowing a liquid coolant countercurrent to the direction of draw through the opening in each of saidplates, except that with which the tubing is first contacted, to surround completely said tubing with the coolant; and, thereafter drawing said tubing through a final draw plate provided with anopening to give the external shape desired in the finished tubing, said plate being set at an angle of about 45 to the line of draw and being substantially immersed in a cooling fluid.

4. In a method for the fabrication of tubing, the steps which include: expressing a heated thermoplastic through a die to form a tube while supporting the tube internally by introducing a fluid thereinto, subjecting said tube while still in a plastic condition from the hot expression step to a preliminary drawing operation, and thereafter drawing said tube through a final draw plate provided with an opening of the exact external shape desired in the finished tube, the plane of said opening being at other than a right angle to the line of draw.

5. A method as set forth in claim 4, in which the plates at any points where the tube and plates 3' would contact in the absence of said coolant.

6. In a method for the fabrication of tubing, the steps which include: pressure extruding heated resinous thermoplastic tubing having a wall thickness that becomes self-sustaining when cooled, supporting said tubing internally by fluid under pressure, and drawing said tubing through a draw plate while still plastic from the extruding step to the desired cross-sectional shape while surrounding said tubing with liquid coolant between the tubing and draw plate during the drawing operation.

7. A method as set forth in claim 6, in which the tubing is given a final circular cross-sectional contour.

8. A methodas'set forth in claim 6, in which the tubing is given a final polygonal cross-sectional contour.

9. In a method for the fabrication of tubing, the steps which include: extruding thermoplas-. tic tubing of annular cross-section, supporting said tubing internally by fluid pressure, and drawing said tubing while still relatively plastic through a draw plate at other than a right angle to the line of draw, said draw plate having an 1 opening of the desired polygonal contour to polygonal cross-sectional contour.

10. In a method for the fabrication of tubing, the steps which include: extruding thermoplastic tubing having a wall thickness that becomes self-,

sustaining when cooled, supporting said tubing internally by fluid pressure, and drawing said tubing while still relatively plastic from the extrusion step to the desired cross-sectional shape through a final draw plate set at other than a right angle to the line of draw.

11. In apparatus for the fabrication of tubing by drawing operations from die extruded thermoplastic, in combination, a preliminary set of 13. In apparatus for the fabrication of tubing by drawing operations from die extruded thermoplastic, in combination, a preliminary draw plate, and a final draw plate spaced therefrom, said final draw plate having an opening to give the exact external shape desired in the finished tubing, the plane of the opening of the final draw plate being at other than a right angle to the line of draw.

14. Apparatus as set forth in claim 13, including means for maintaining liquid coolant between the tubing and the adjacent portions of the draw plates.

15. In apparatus for the fabrication of tubing by drawing operations from die extruded thermoplastic, in combination, a preliminary draw plate, and final draw plate means spaced therefrom to give the tubing the final external shape desired, said final draw plate means including draw plate surfaces contacting the tubing in such manner that one point on the periphery of the tubing is no longer in contact with the final draw plate means when another point on the periphery of the tubing opposite the first mentioned point is in contact with the final draw plate meansso that a, line through the said points of contact of draw plate surfaces with the tubing is at an angle other than 90 to the line of draw.

16. In' apparatus for the fabrication of tubing by drawing operations from die extruded thermoplastic, in combination, a preliminary draw plate, and final draw plate means spaced therefrom to give the tubing the final external shape desired, said final draw plate means including draw plate surfaces contacting the tubing in such manner that said final draw plate means is in contact with one point on the periphery of the tubing in advance of its contact with another point on the periphery of the tubing, both of said points lying in a plane at 90 to the tubing, so that a line through the said points oftcontact of draw plate surfaces with the tubing is at an angle other than 90 to the line of draw.

CHARLES E SLAUGHTER. 

